1. Field of the Invention
The present invention relates to a musical tone synthesizing apparatus which synthesizes musical tones by simulating sounds of acoustic musical instruments.
2. Prior Art
The musical tone synthesizing apparatuses conventionally known are designed to synthesize musical tones by using simulation models which simulate sounding mechanisms of the acoustic musical instruments. Among those musical tone synthesizing apparatuses, some of them are designed to simulate sounds of wind instruments. This kind of musical tone synthesizing apparatus simulating the sounding mechanism of the wind instrument is mainly configured by an excitation circuit and a resonance circuit, which are connected together. Herein, the excitation circuit is provided to simulate operations of a mouthpiece portion, while the resonance circuit is provided to simulate resonance characteristics of a resonance tube.
Some wind instruments such as the saxophone, trumpet and clarinet have a resonance tube which has a cone-like shape. This resonance tube can be simulated by a simulation model of tube constructed by a plurality of cylindrical tubes, each having a different diameter, which are assembled together. Each of those cylindrical tubes can be electronically simulated by a pair of a waveguide and a junction. Herein, the waveguide, at least containing a delay circuit, is a bi-directional transmission circuit, while the junction is a connection circuit. When simulating the operations of the above-mentioned tube, there are provided plural pairs of waveguide and junction, which are connected together in a cascade-connection manner. The musical tone synthesizing apparatus using the waveguide and junction is disclosed in Japanese Patent Laid-Open Publication No. 63-40199, for example.
In order to simulate the propagation loss of the air waves which are reflected at the terminal portion of the resonance tube, the waveguide utilizes a multiplier and a low-pass filter as well as the delay circuit. A manner of the reflection of the air waves depends upon the tone color, so that a `reflection coefficient` is introduced to represent the manner of reflection to be selected. As the reflection coefficient, the multiplication coefficient and cut-off frequency are supplied to the multiplier and low-pass filter respectively.
An envelope waveform of the musical tone to be synthesized by the musical tone synthesizing apparatus can be changed by changing an absolute value of a reflection coefficient .gamma.. In order to obtain the envelope waveform as shown in FIG. 11A, the absolute value of the reflection coefficient .gamma. is set at `1`. In FIG. 11A, the envelope waveform is rapidly rising (in other words, the envelope waveform has a sharp attack portion) after performance-input data (i.e., performance information, used for the production of the musical tone, which represents the breath pressure applied to the mouthpiece of the wind instrument, for example) is supplied to the musical tone synthesizing apparatus; and then, the envelope waveform is gradually decaying (in other words, the envelope waveform has a dull decay portion) after the supply of the performance-input data is stopped. In order to obtain an envelope waveform as shown in FIG. 11B, the absolute value of the reflection coefficient .gamma. is reduced. In FIG. 11B, the envelope waveform is gradually rising after the performance-input data is supplied to the musical tone synthesizing apparatus; and then, the envelope waveform is rapidly decaying after the supply of the performance-input data is stopped.
Recently, some demands are raised such that certain musical tones, which cannot be obtained by the existing acoustic musical instruments, are synthesized. However, the musical tone synthesizing apparatuses conventionally known cannot satisfy the above-mentioned demands because the reflection coefficient should be constant during the supply of the performance-input data. In other words, a changing manner of the envelope waveform depends upon the reflection coefficient, so that while the reflection coefficient is remained constant, the envelope waveform cannot be arbitrarily changed. In short, there is a problem that the conventional musical tone synthesizing apparatus cannot synthesize the musical tone having the envelope waveform, to be arbitrarily changed, in which both of the attack portion and decay portion are intentionally made sharp or dull, for example.
Meanwhile, another type of musical tone synthesizing apparatus conventionally known has an automatic key-scaling function by which a key-scaling operation is automatically performed for the delay-feedback-type sound source. This delay-feedback-type sound source is disclosed in Japanese Patent Laid-Open Publication No. 58-48109, for example. An example of this sound source is shown in FIG. 12.
In FIG. 12, an excitation-waveform generating portion 101 stores fundamental musical-tone waveforms so as to selectively output one musical-tone waveform. This excitation-waveform generating portion 101 receives signals "WAVE", "TOUCH" and "KON". The signal WAVE is used to designate the musical-tone waveform to be selectively read out from among the musical-tone waveforms stored in the excitation-waveform generating portion 101; the signal TOUCH represents an intensity of depressing the key of the keyboard; and the signal KON designates a timing to output the read musical-tone waveform. Thus, an initial musical-tone waveform is outputted from the excitation-waveform generating portion 101 and is supplied to an adder 102. The adder 102 adds the initial musical-tone waveform to an output signal of a variable amplifier 103. Then, a result of the addition performed by the adder 102 is supplied to a filter 104. The filter 104 receives a signal FC which is used to control a filter coefficient. Thus, on the basis of the filter coefficient controlled by the signal FC, the filter 104 effects a certain filtering operation on the result of addition produced by the adder 102. Through the filtering operation, a desired frequency characteristic is imparted to the musical tone. An output signal of the filter 104 is outputted as a musical tone signal and is also fed back to the adder 102 through a feedback loop consisting of the variable amplifier 103 and a variable delay circuit 105.
The variable amplifier 103 is provided to determine a loop gain. In other words, this variable amplifier 103 is provided to attenuate a level of a signal to be fed back to the adder 102. A gain `a` of the variable amplifier 103 is controlled responsive to a gain signal. This gain `a` is affected by the characteristic of the filter 104, whereas if the gain `a` is set at `1` (i.e., 0 dB) at the band-pass range of the filter 104, the gain `a` should be smaller than `1` while an decaying sound is producing. The variable amplifier 103 multiples an output signal of the variable delay circuit 105 by the gain `a` to produce a feedback signal which is then supplied to the adder 102. The variable delay circuit 105 has a delay time DLY which is controlled responsive to a delay-amount signal supplied thereto, so that the output signal of the filter 104 is delayed by the delay time DLY. The pitch of the musical tone is determined by the delay time of the variable delay circuit 105. Strictly speaking, the pitch of the musical tone is determined by the delay time of the variable delay circuit 105 as well as the delay time which is caused by the filtering operation performed by the filter 104.
When producing the musical tone having a higher pitch, it is necessary to reduce the amount of delay to be applied to the musical tone signal. On the other hand, when producing the musical tone having a lower pitch, it is necessary to increase the amount of delay to be applied to the musical tone signal. As compared to the lower-pitch musical tone, the production of the higher-pitch musical tone requires a more number of times by which the initial musical-tone waveform is repeatedly passing through the feedback loop. This means that as compared to the lower-pitch musical tone, the production of the higher-pitch musical tone requires a more number of times by which the musical tone signal is repeatedly multiplied by the gain `a` by the variable amplifier 103. If the gain is constant, as compared to the lower-pitch musical tone, the higher-pitch musical tone has a shorter period of time in which the production of the musical tone is sustained.
For example, when producing the musical tone at 440 Hz, the multiplication using the gain is repeatedly performed by four-hundred-and-forty times in one second. Similarly, when producing the musical tone at 880 Hz, the multiplication is performed by eight-hundred-and-eighty times in one second. Thus, when producing those musical tones by using the same initial musical-tone waveform and the same gain, an decay rate for the musical tone of 880 Hz is the double of an decay rate for the musical tone at 440 Hz. In other words, every time the pitch is raised up by one octave, the decay rate is doubled.
In order to solve the problem caused by the variation of the decay rate, some methods are employed. According to a first method, a break point (i.e., a split point between the registers) is set such that the sustaining time to continuously produce the musical tone is not extremely changed with respect to each of the musical tones to be produced. This method controls the sustaining time of each musical tone to be almost constant, within the whole register, in terms of the listening comprehension. A second method uses a data table by which the value of the gain is changed by each musical tone to be produced so that the sustaining time will be constant.
In the first method described above, however, it is necessary to change a timing to set the break point with respect to each musical tone. Therefore, this method requires a complex processing. Even in the second method, the values of the gain should be stored in the data table with respect to each musical tone. Thus, a storage capacity to be required in the second method should be increased, which will result in an increase of the cost for manufacturing the apparatus. Further, the conventional apparatus as shown in FIG. 12 is configured such that a predetermined value is set for the gain in response to the musical tone to be designated. Thus, the conventional apparatus is disadvantageous in that a performer cannot freely set the sustaining time of the musical tone.